The present invention relates, in general, to electrical power supplies and, in particular, to a new and useful constant current source useful in field contact inputs where from 24-125 volts DC is required at typically 10 mA.
Circuit designs are necessary for power and process control industries for field contact input sensing and for providing necessary information to a digital processing/control system.
Customers in the power and process control industries typically require a particular current flow at a specific voltage which a customer supplied field input contact must deliver in order to maintain proper contact cleaning. Typically 10 mA are provided at 24 VDC 48 VDC or 125 VDC.
Previously, either system suppliers have offered only one voltage at the required current or they have different assemblies made to allow the same required current at different voltages. Previous systems have also provided an option which is selectable by using jumpers, to change the resistance in the circuit. The first-mentioned technique does not allow the supply to satisfy a variety of customers. The next two techniques require costly paperwork and job system processing, to keep track of the type of inputs for every job. The second technique also requires costly stocking of different modules or the last-mentioned technique requires the mounting of all components on the module and manual intervention to provide the proper jumping in and out of components based on the voltage requirement.
FIG. 1 shows the previous power supply wherein a jumper A and B is added for a 24-volt option and a jumper B is added for a 48-volt option, the 125 volt-option being effected where no jumpers are used.